Gronouski was born in Dunbar, Wisconsin, the son of Mary (Riley) and John Austin Gronouski. He was of Polish and Irish descent.[1] He graduated from the University of Wisconsin in 1942, and he joined the military during World War II. Gronouski served as a navigator in the Air Force until October 1945. Gronouski married the former Mary Louise Metz on January 24, 1948. They had two daughters, Stacy Ann Jennings and Julia Kay Glieberman.

John Gronouski, Ambassador to Poland, meets his friends Wiesław and Longina Czajkowski with Chairman of the Polish-American Chamber of Commerce in Detroit, Chester A. Kozdroj and his wife Helena at the U.S. Embassy in Warsaw, September, 1966.

In 1960, he became the Wisconsin state commissioner of taxation, and he supported John F. Kennedy for President. In 1963 Gronouski was appointed Postmaster General, the first Polish-American Cabinet officer. As Postmaster General, Gronouski promoted the original five-digit zip code system, and worked to the end racial discrimination against postal employees.[3] After he left the Cabinet on November 2, 1965, President Lyndon B. Johnson appointed him to be Ambassador to Poland.